1. Field of the Invention
The present invention relates generally to an apparatus and a method for manufacturing semiconductor devices, and specifically to an apparatus and a method for forming silicon oxide films.
2. Discussion of the Background
In the conventional method for manufacturing a semiconductor device, TEOS gas has widely been used for forming silicon oxide films. In these methods, silicon wafers are placed in a quartz chamber which has been heated to about 700.degree. C., and TEOS gas is introduced into the quartz chamber to form silicon oxide films on the surface of the silicon wafers by thermal decomposition. TEOS is the abbreviation of tetraethyl ortho-silicate, represented by the chemical formula, Si(OC.sub.2 H.sub.5).sub.4.
Referring to drawings, FIG. 3 is a schematic diagram illustrating the construction of a conventional semiconductor manufacturing apparatus for forming silicon oxide (SiO.sub.2) films. Specifically, this is a reduced pressure CVD apparatus for forming silicon oxide films. As FIG. 3 shows, this manufacturing apparatus comprises a quartz chamber 1, and a heater 2 for heating the quartz chamber 1. The quartz chamber 1 accommodates silicon wafers 3 and a quartz boat 4 on which the silicon wafers are placed. The quartz chamber 1 is connected to a TEOS gas supply source and a nitrogen gas supply source through pneumatic valves 5a, 5b and mass flow controllers 6a, 6b, respectively. An exhaust pipe 7, a main valve 8a, a sub-valve 8b and a vacuum pump 9 are also connected to the quartz chamber 1.
FIG. 4 is a flow diagram showing a conventional method for forming silicon oxide films when silicon oxide films are formed using the manufacturing apparatus described above.
Referring to FIGS. 3 and 4, the conventional method of forming silicon oxide films will be described. First, a boat 4 holding silicon wafers 3 is loaded into a quartz chamber 1 under atmospheric pressure. The valve 5b for introducing nitrogen gas (N.sub.2) is opened, and nitrogen gas is introduced into the quartz chamber 1 at a flow rate of 2 l/min (liter per minute) through a mass flow controller 6b for controlling the flow rate (see FIG. 4, step F11, Boat load).
Next, the flow rate of nitrogen gas is adjusted to 200 cc/min using the mass flow controller 6b for adjusting the flow rate of nitrogen gas, and the sub-valve 8b for exhausting is opened to evacuate the quartz chamber 1 slowly. When the pressure in the chamber 1 has been lowered to 266 Pa (20 Torr) or less, the valve 5b for introducing nitrogen gas is closed, and the evacuation is continued until the pressure reaches 133 Pa (1 Torr) or less. Thereafter, the main valve 8a for exhausting is opened, and evacuation is further continued until the pressure is lowered to 133.times.10.sup.-3 Pa (1.times.10.sup.-3 Torr) or less (see FIG. 4, step F12, Evacuation).
Then, the sub-valve 8b and the main valve 8a for exhausting are closed, the pressure in the quartz chamber 1 is maintained, and leakage is checked. When the pressure in the chamber stays at 13.3 Pa (0.1 Torr) or less, leakage is deemed not to occur, and the sub-valve 8b and the main valve 8a are opened again to evacuate the quartz chamber 1 (see FIG. 4, step F13, Leakage check).
Next, the valve 5a for introducing TEOS gas is opened, and the flow rate of TEOS gas is adjusted to 100 cc/min. using the mass flow controller 6a to introduce the TEOS gas into the quartz chamber 1. The quartz chamber 1 is maintained at a pressure of 173 Pa (1.3 Torr) and a temperature of 670.degree. C. during oxide film formation (see FIG. 4, step F14, TEOS introduction and film formation).
After the oxide film has been formed, the valve 5a for introducing TEOS gas is closed, and the valve 5b for introducing nitrogen gas is opened to purge the gas in the quartz chamber 1 with nitrogen gas (see FIG. 4, step F15, N.sub.2 introduction).
After the completion of purging the TEOS gas, the flow rate of nitrogen gas is adjusted to 500 cc/min. using the mass flow controller 6b for adjusting the flow rate of nitrogen gas. The main valve 8a for exhausting is closed, and the pressure in the quartz chamber 1 is adjusted to about 665 Pa (5 Torr).
Next the sub-valve 8b is closed to adjust the pressure of the nitrogen gas to about 3990 Pa (30 Torr). Thereafter, the setting of the mass flow controller 6b is slowly raised to return the pressure of the nitrogen gas to normal pressure (see FIG. 4, step F16, Returning to Atmospheric pressure).
Finally, the boat 4 is unloaded from the quartz chamber 1 (see FIG. 4, step F17, Boat unloading).
In the conventional apparatus and method for forming silicon oxide films on semiconductor devices as described above, the majority of TEOS gas introduced reaches the vacuum exhaust pipe 7 without being completely decomposed thermally, and deposits are formed in the pipe 7. These deposits cause out-gassing during the formation of the silicon oxide films, which in turn forms particles and deposits on the surface of the silicon wafer, lowering the reliability of the products.
Referring to the drawings, in the semiconductor manufacturing apparatus of FIG. 3, the majority of TEOS gas introduced into the chamber 1 reaches the vacuum exhaust pipe 7 without being completely decomposed, where it is cooled and the deposits are formed as shown in FIG. 5.
In the silicon oxide film forming process, nitrogen gas is introduced after film formation for a specified time, to return the chamber to normal pressure, as shown in FIG. 6(a). At this time, pressure in the chamber 1 rises to a little higher than the atmospheric pressure. Therefore, in the following boat unloading process, a back flow of the gas from the vacuum exhaust pipe 7 towards the quartz chamber 1 is caused, as shown in FIG. 6(b). At this time, fine particles or out-gassing from the deposits which have not yet reacted reach the quartz chamber 1 and form particles. In order to decrease the formation of the particles, therefore, it is necessary that the non-reacted gas will not easily deposit in the exhaust pipe 7, and that the generation of fine particles or out-gassing is decreased by lowering the amount of the non-reacted gas.